Diffusion olefins

Ideal reaction conditions of high Isobutane concentration, and isobutane diffusion into the acid phase at a rate greater than the olefin diffusion rate, result in high quality alkylate product. Ester formation, exclusive of those formed from feed contaminants,... 

The proposed mechanism for the isomerization of n-alkanes on bifunctional catalysts (60,61) is presented in Figure 14. From this mechanism an equilibrium between paraffins and olefins is established on the metal function. Then the olefins diffuse towards the Bronsted sites, where they become protonated and rearranged to give the branched carbenium ions. This, which is the rate controlling step, is followed by the desorption and hydrogenation, to yield the branched paraffins. 

A.E. Tugtas, P. Cavdar and B. Calli, Continuous flow membrane-less air cathode microbial fuel cell with spunbonded olefin diffusion layer. Bioresource Technol. 102,2011, 10425-10430. 

Olefins are hydrogenated very easily, unless highly hindered, over a variety of catalysts. With active catalysts, the reaction is apt to be diffusion limited, since hydrogen can be consumed faster than it can be supplied to the catalyst surface. Most problems connected with olefin hydrogenation involve some aspect of regio- or stereoselectivity. Often the course of reduction is influenced greatly by the catalyst, by reaction variables, and by hydrogen availability at the catalyst surface. 

High thermodynamic selectivity (7) demands that the initially formed cis olefin be displaced rapidly relative to its saturation or to its isomerization. As the reaction nears completion and the acetylene concentration diminishes, its effectiveness in displacing olefin will diminish and selectivity will fall. Displacement by acetylene is also impeded through depletion of acetylene in the vicinity of the catalyst owing to intra- or interpartile diffusion resistance (53a). A change in a reaction parameter thus can have different influences... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

The enzyme catalysed epoxidation of alpha-olefins like 1-octene with oxygen to the optically active epoxide provides an interesting example of a four-phase system (de Bont et al., 1983). The hold-up of the organic phase may be 2-4 % and the presence of biosurface active agents leads to the creation of a large liquid-liquid interfacial area the liquid droplet size becomes smaller than the gas-liquid diffusion film thickness. 

In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 1) complex of bromide with o-CA, in which the anion is located over the center of the C - C bond of the acceptor moiety (Fig. 15b) and Br - C contacts are shortened by as much as 0.6 A relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 1 complex [(Br )2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropyl- or tetrabutylammonium salts of the same anion are employed, the (1 1) complexes [Br ,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-... 

Bromination is less stereoselective, and the reactions of cis- and trans-olefins tend to be stereoconvergent. The stereospecific formation of the mixed bromoadducts in protic media, such as methanol or acetic acid, could be interpreted in the light of the recent finding (Ruasse et al., 1991) that these solvents assist the formation of the ionic intermediate nucleophilically. If a solvent molecule is close to the cationic part of the transition state in the rate-limiting step, the intermediate can be trapped by this solvent molecule in a necessarily trans mode with respect to the first bromine, before the two components of the ion-pair diffuse away from each other (15). This would... 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

A quantitative model requires knowledge of the diffusivity under reaction conditions and of the intrinsic activities for toluene disproportionation and xylene isomerization. While these are not easily obtained, the methodology has been worked out for the case of paraffin and olefin cracking (5). So far, we have obtained an approximate value for the diffusivity, D, of o-xylene at operation conditions from the rate of sorptive o-xylene uptake at lower temperature and extrapolation to 482°C (Table V). 

Sampling in inverse coannular diffusion flames [62] in which propene was the fuel has shown the presence of large quantities of allene. Schalla et al. [57] also have shown that propene is second to butene as the most prolific sooter of the n-olefins. Indeed, this result is consistent with the data for propene and allene in Ref. 72. Allene and its isomer methylacetylene exhibit what at first glance appears to be an unusually high tendency to soot. However, Wu and Kem [111] have shown that both pyrolyze relatively rapidly to form benzene. This pyrolysis step is represented as alternate route C in Fig. 8.23. 

OH species, 30 249 olefin, 25 138, 139 on oxide semiconductors, 7 47 of oxygen, 27 192 relative contents of various forms of in absence of illumination, 23 161-164 on illumination, 23 164-170 SCF-LCAO-MO procedure, 25 35, 36 of simple molecules, 34 166-174 sites for, 26 360, 361 uniformity of, 26 361, 362 spectroscopy, 25 198-202 of spillover hydrogen, the kinetics of, 34 7 surface diffusion as rate-determining step, 34 7... 

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